Fouling can be defined as the accumulation of unwanted matter on heat transfer surfaces. This deposition can be very costly in refinery and petrochemical plants since it increases fuel usage, results in interrupted operations and production losses and increases maintenance costs.
Deposits are found in a variety of equipment: preheat exchangers, overhead condensers, furnaces, heat exchangers, fractionating towers, reboilers, compressors and reactor beds. These deposits are complex but they can be broadly characterized as organic and inorganic. They consist of metal oxides and sulfides, soluble organic metals, organic polymers, coke, salt and various other particulate matter.
The chemical composition of organic foulants is rarely identified completely. Organic fouling is caused by insoluble polymers which sometimes are degraded to coke. The polymers are usually formed by reactions of unsaturated hydrocarbons, although any hydrocarbon can polymerize. Generally, olefins tend to polymerize more readily than aromatics, which in turn polymerize more readily than paraffins. Trace organic materials containing Hetero atoms such as nitrogen, oxygen and sulfur also contribute to polymerization.
Polymers are generally formed by free radical chain reactions. These reactions, shown below, consist of two phases, an initiation phase and a propagation phase. In Reaction 1, the chain initiation reaction, a free radical represented by R., is formed (the symbol R.multidot. can be any hydrocarbon). These free radicals, which have an odd electron, act as chain carriers. During chain propagation, additional free radicals are formed and the hydrocarbon molecules (R) grow larger and larger (see Reaction 4), forming the unwanted polymers which accumulate on heat transfer surfaces.
Chain reactions can be triggered in several ways. In Reaction 1, heat starts the chain. Example: When a reactive molecule such as an olefin or a diolefin is heated, a free radical is produced. Another way a chain reaction starts is shown in Reaction 3. Metal ions initiate free radical formation here. Accelerating polymerization by oxygen and metals can be seen by reviewing Reactions 2 and 3.
As polymers form, more polymers begin to adhere to the heat transfer surfaces. This adherence results in dehydrogenation of the hydrocarbon and eventually the polymer is converted to coke.
1. Chain Initiation EQU R-H.fwdarw.R.multidot.+H.multidot.
2. Chain Propagation EQU a. R.multidot.+O.sub.2 .fwdarw.R--O--O.multidot. EQU b. R-O-O.multidot.+R'-H.fwdarw.R.multidot.+R--O--O--H EQU c. R'.multidot.+C=C.fwdarw.R'-C-C.multidot..fwdarw.Polymer
3. Chain Initiation EQU a. Me.sup.++ +RH.fwdarw.Me.sup.+ R.multidot.+H.sup.+ EQU b. Me.sup.++ +R--O--O--H.fwdarw.Me.sup.+ R--O--O.multidot.+H.sup.+
4. Chain Termination EQU a. R.multidot.+R.multidot..fwdarw.R-R' EQU b. R.multidot.+R--O--O.multidot..fwdarw.R--O--O--R
In refineries, deposits usually contain both organic and inorganic compounds. This makes the identification of the exact cause of fouling extremely difficult. Even if it were possible to precisely identify every single deposit constituent, this would not guarantee uncovering the cause of the problem. Assumptions are often erroneously made that if a deposit is predominantly a certain compound, then that compound is the cause of the fouling. In reality, oftentimes a minor constituent in the deposit could be acting as a binder, a catalyst, or in some other role that influences actual deposit formation.
The final form of the deposit as viewed by analytical chemists may not always indicate its origin or cause. Before openings, equipment is steamed, water-washed, or otherwise readied for inspection. During this preparation, fouling matter can be changed both physically and chemically. For example, water-soluble salts can be washed away or certain deposit constituents oxidized to another form.
In petrochemical plants, fouling matter is often organic in nature. Fouling can be severe when monomers convert to polymers before they leave the plant. This is most likely to happen in streams high in ethylene, propylene, butadiene, styrene and other unsaturates. Probable locations for such reactions include units where the unsaturates are being handled or purified, or in streams which contain these reactive materials only as contaminants.
Even through some petrochemical fouling problems seem similar, subtle differences in feedstock, processing schemes, processing equipment and type of contaminants can lead to variations in fouling severity. For example, ethylene plant depropanizer reboilers experience fouling that appears to be primarily polybutadiene in nature. The severity of the problem varies significantly from plant to plant, however. The average reboiler run length may vary from one to two weeks up to four to six months (without chemical treatment).
Although it is usually impractical to identify the fouling problem by analytical techniques alone, this information combined with knowledge of the process, processing conditions and the factors known to contribute to fouling, are all essential to understanding the problem.
There are many ways to reduce fouling both mechanically and chemically. Chemical additives often offer an effective anti-fouling means; however, processing changes, mechanical modifications equipment and other methods available to the plant should not be overlooked.
Antifoulant chemicals are formulated from several materials: some prevent foulants from forming, others prevent foulants from depositing on heat transfer equipment. Materials that prevent deposit formation include antioxidants, metal coordinators and corrosion inhibitors. Compounds that prevent deposition are surfactants which act as detergents or dispersants.
Different combinations of these properties are blended together to maximize results for each different application. These "poly-functional" antifoulants are generally more versatile and effective since they can be designed to combat various types of fouling that can be present in any given system.
Research indicates that even very small amounts of oxygen can cause or accelerate polymerization. Accordingly, anti-oxidant-type antifoulants have been developed to prevent oxygen from initiating polymerization. Antioxidants act as chain-stoppers by forming inert molecules with the oxidized free radical hydrocarbons, in accordance with the following reaction:
Chain Termination EQU ROO.multidot.Antioxidant.fwdarw.ROOH+Antioxidant (H)
Surface modifiers or detergents change metal surface characteristics to prevent foulants from depositing. Dispersants or stabilizers prevent insoluble polymers, coke and other particulate matter from agglomerating into large particles which can settle out of the process stream and adhere to the metal surfaces of process equipment. They also modify the particle surface so that polymerization cannot readily take place.
Antifoulants are designed to prevent equipment surfaces from fouling. They are not designed to clean up existing foulants. Therefore, an antifoulant should be started immediately after equipment is cleaned. It is usually advantageous to pretreat the system at double the recommended dosage for two or three weeks to reduce the initial high rate of fouling immediately after startup.
The increased profit possible with the use of antifoulants varies from application to application. It can include an increase in production, fuel savings, maintenance savings and other savings from greater operating efficiency.
There are many areas in the hydrocarbon processing industry where antifoulants have been used extensively; the main areas of treatment are discussed below.
In a refinery, the crude unit has been the focus of attention because of increased fuel costs. Antifoulants have been successfully applied at the exchangers; downstream and upstream of the desalter, on the product side of the preheat train, on both sides of the desalter makeup water exchanger and at the sour water stripper.
Hydrodesulfurization units of all types experience preheat fouling problems. Among those that have been successfully treated are reformer pretreaters processing both straight run and coker naphtha, desulfurizers processing catalytically cracked and coker gas oil, and distillate hydro-treaters. In one case, fouling of a Unifiner stripper column was solved by applying a corrosion inhibitor upstream of the problem source.
Unsaturated and saturated gas plants (refinery vapor recovery units) experience fouling in the various fractionation columns, reboilers and compressors. In some cases, a corrosion control program combined with an antifoulant program gave the best results. In other cases, an application of antifoulants alone was enough to solve the problem.
Cat cracker preheat exchanger fouling, both at the vacuum column and at the cat cracker itself, has also been corrected by the use of antifoulants.
The two most prevalent areas for fouling problems in petrochemical plants are at the ethylene and styrene plants. In an ethylene plant, the furnace gas compressors, the various fractionating columns and reboilers are subject to fouling. Polyfunctional antifoulants, for the most part, have provided good results in these areas. Fouling can also be a problem at the butadiene extraction area. Both antioxidants and polyfunctional antifoulants have been used with good results.
In the different design butadiene plants, absorption oil fouling and distillation column and reboiler fouling have been corrected with various types of antifoulants.
Chlorinated hydrocarbon plants, such as VCM, EDC and perchloroethane and tri-chloroethane have all experienced various types of fouling problems. The metal coordinating/antioxidant-type antifoulants give excellent service in these areas.